Polyether polyols and polyester polyols are well-known polymers which can be further polymerized with organic polyisocyanates to prepare polyurethanes. Polyether polyols are prepared by the reaction of hydroxy-containing hydrocarbons, such as an aromatic or aliphatic diol or triol, and epoxides, e.g., ethylene oxide and propylene oxide. Polyester polyols are prepared by the reaction of polyacids, such as adipic or terephthalic acid, or esters of polyacids, such as dimethyl adipate or dimethyl terephthalate with dihydroxy-containing hydrocarbons, such as aromatic and aliphatic diols and triols. Some poly(alkylene carbonate) polyol properties resemble polyester polyol properties while other properties resemble polyether polyols.
It is known to prepare polycarbonates from aliphatic dihydroxyl compounds either by a process of phosgenation in which hydrogen chloride is liberated or bound by bases, such as pyridine or quinoline, or by a process of transesterification with carbonic acid esters of aleohols or phenols, preferably diphenylcarbonate, optionally with the aid of transesterification catalysts. In either case, it is essential to use phosgene or a mixture of carbon monoxide and chlorine as the source of carbonic acid. Technical processes which involve the preparation and handling of phosgene are difficult and costly on account of considerable safety risks involved and the high cost of materials due to corrosion. To this are added ecological problems since either the spent air is contaminated with hydrogen chloride or the effluent water is contaminated with sodium chloride.
Polycarbonates produced by these methods, using dihydrocarbyl compounds, may have a functionality of less than two due to inadequate or incomplete esterification or transesterification which often prevents the products from forming high molecular weight polymers in subsequent reactions.
U.S. Pat. Nos. 3,248,414, 3,248,415 and 3,248,416 to Stevens disclosed the preparation of poly(alkylene carbonate) polyols from
(1) carbon dioxide and 1,2-epoxides;
(2) cyclic carbonates such as ethylene carbonate; or
(3) cyclic carbonates and a 1,2-epoxide.
A minor amount of a polyol is employed therein as an initiator. The reaction is usually conducted in the presence of a metal carbonate, metal hydroxide, trisodium phosphate or tertiary amine.
Poly(alkylene carbonate) polyols have also been prepared by polymerization of ethylene carbonates under pressure using basic catalysts and a minor amount of glycol as initiator as disclosed in U.S. Pat. No. 4,105,641 to Buysch et al. These products are low in carbonate and high in ether group concentration due to decomposition of the ethylene carbonate. In the Stevens patents discussed hereinbefore, a poly(alkylene carbonate) polyol derived from ethylene carbonate and monoethylene glycol was exposed to temperatures of 160.degree. C. at 2 mm Hg of pressure to remove unreacted ethylene carbonate. In U.S. Pat. No. 3,379,693, Hostetler removed unreacted ethylene carbonate from products similar to poly(alkylene carbonate) polyols by heating to about 130.degree. C. under a pressure of 1-5 mm Hg. In U.S. Pat. No. 3,896,090 to Maximovich, ethylene carbonate was reacted with diethylene glycol and the reaction product treated under reduced pressure to remove the unreacted ethylene carbonate and diethylene glycol.
Several workers have prepared poly(alkylene carbonate) polyols and related materials by controlling the equilibrium between the reaction materials of a diol and alkylene carbonate and the products of a poly(alkylene carbonate) polyol and monoethylene glycol. The reaction is controlled by the removal of monoethylene glycol.
In U.S. Pat. No. 3,133,113 to Malkemus, ethylene carbonate and diethylene glycol were reacted at 125.degree. C. to 130.degree. C. under a reduced pressure of 100 mm Hg in the presence of certain catalysts with concurrent removal of monoethylene glycol as a distillate. The catalyst employed was a mixed zinc borate-alkaline earth metal oxide catalyst. This was followed by removal of starting material. The Malkemus procedure is plagued by the presence of volatile ethylene carbonate which condenses as a solid throughout the system causing severe plugging and reducing ethylene carbonate conversion while monoethylene glycol is being removed. This process requires large excesses of ethylene carbonate.
In U.S. Pat. No. 3,313,782 to Springmann et al., this process was further studied under reduced pressure in the presence of catalysts, and limits on the reaction conditions were set. The reaction temperatures must be lower in this process than the boiling point of the alkylene carbonate, but still high enough to distill off the monoethylene glycol formed.
U.S. Pat. No. 4,131,731 to Lai et al. used stage reductions in pressure during the reaction of alkylene carbonate with a diol. The final stage of the Lai et al. process is intended to remove monoethylene glycol. The patentees characterized their reaction conditions by stating that the alkylene carbonate must have a boiling point 4.9.degree. C. greater than monoethylene glycol. The chemistry based on the above equilibrium was improved upon by U.S. Pat. No. 4,105,641 to Buysch et al. where the reactions were carried out in a solvent (e.g., cumene) capable of removing monoethylene glycol as an azeotrope with the solvent.
Heretofore, the molecular weights of poly(alkylene carbonate) polyols from alkylene carbonates have been controlled by either the stoichiometry of the reactants, that is, higher alkylene carbonate to initiator ratios for higher molecular weights, or the removal of monoethylene glycol from the reaction mixture with an ethylene carbonate to initiator equivalent ratio of about 1. Catalysts are used in most cases since reaction rates are very slow in the absence of a catalyst. When high alkylene carbonate to initiator ratios are used to make higher molecular weight poly(alkylene carbonate) polyols, reaction rates drop severely as higher conversions are approached. In these cases, long reaction times are required and the products are contaminated by unreacted alkylene carbonate. If temperatures are increased to increase the rate, product decomposition occurs with CO.sub.2 loss.
In view of the deficiencies of the conventional poly(alkylene carbonate) polyahls, it would be highly desirable to provide poly(alkylene carbonate) polyahls having improved physical and chemical properties.